Phase change or resistance change materials, such as germanium telluride (GeTe), exhibit two distinct resistivity values, different by several orders of magnitude, when they transition between the crystalline and amorphous states. Advances in micro- and nano-fabrication technology have made it possible to integrate phase change materials into digital non-volatile memory cells and more recently in RF switches. Compared to switches using other solid-state or MEMS technologies, phase change switches (a.k.a vias) offer smaller loss for similar switch sizes with competitive linearity and power handling capability. Recent research also demonstrated phase change switches with incredibly high power handling capability and good linearity, making them a strong candidate for high-power RF applications.
The basic principle of operation for phase change switches is the thermal transition of the phase change layer between crystalline (low-resistance) and amorphous (high-resistance) states. When used as RF switches, the transition between these two states is typically achieved using either of the two Joule heating methods: direct or indirect heating, both with the application of current (or voltage) pulses. In the direct heating approach, the current is drawn through the phase change via itself, whereas in the indirect heating scheme, a heater line is placed adjacent to the switch and is electrically isolated from the RF path but thermally coupled to the via.
The challenge with direct heating structures in the past, specifically for RF applications, is that the DC and RF thermal path are not isolated and therefore, the heater cannot be designed independently. For the heater, high electrical resistance is needed, whereas for the RF path the electrical and contact resistance should be as low as possible. In the indirect heating scheme, a separate conductive path is used for the heater. However, indirect heating schemes have several issues: 1) higher power is required to phase transition the phase change layer; 2) the RF connection to the phase change layer is a local cold spot, resulting in an increased ON resistance. Running higher current through the heater to increase the temperature at the RF/phase change layer contact increases the power consumption and may reduce reliability. The present disclosure addresses these issues using new switch structures.
This section provides background information related to the present disclosure which is not necessarily prior art.